This bug exists for people who are used to "jump" the whistle motion, i.e. you time the space bar to start flying taking into account the network delay, so that you don't wait for the mount to appear. Two ways to mitigate it:

1. Don't jump the whistle motion, let it finish and let the mount appear before you take off.
2. Even if you jump the motion, the speed is corrected the moment you stop and move again. So a quick release-then-press on the movement key will correct its speed.
They need to fix it.

I don't "jump" the animation, and I've had it happen to me after I've already started flying, at normal speed.

In fact, I've got video of the latter.

Don't spawn the mount until after the animation finishes if it's such a big deal. EQ2 actually got rid of it's mounting animation YEARS ago because it caused issues like this.